A clipper circuit supplies an output signal that, over a prescribed range of an input signal supplied thereto, responds linearly to that input signal. As the input signal swings beyond a boundary of that prescribed range, however, the output signal of the clipper circuit does not change from its value at the boundary. The use of diodes in clipper circuitry is well known. Half-wave and full-wave rectifier circuits are particular examples of clipper circuitry using diodes, which examples are familiar to persons skilled in the electronics art.
Rectifying circuits are generally capable of wide bandwidth operation and are unconditionally stable. However, the use of diodes is problematic when it is necessary and/or desirable to rectify, or otherwise clip, AC signals whose amplitude may be a fraction of a volt since diodes have a forward voltage drop (VF) of approximately 0.6 volt which must be exceeded before they conduct. Also, diodes suffer the disadvantage of being highly non-linear for AC input signal levels near the "knee" of the diode characteristic curve. This limits the usefulness of clipper and rectifying circuits using diodes to relatively high amplitude level signals of relatively limited dynamic range. Therefore, where it is required to rectify an AC signal which may range from a few millivolts to a few volts the diode circuits discussed above, which are also referred to as "feed forward" type circuits, are not suitable.
To overcome the problem with "feed forward" clipper (e.g., diode rectifying) circuits, rectifier or absolute value circuits capable of linear operation down to small AC signal voltages generally employ feedback techniques to reduce the effect of diode nonlinearities. An example of such a circuit is shown in U.S. Pat. No. 4,564,814 issued Jan. 14, 1986 to Miura et alii and titled FULL-WAVE RECTIFIER USING AN OPERATIONAL AMPLIFIER.
Referring specifically to FIGS. 3 and 4 of the Miura et alii reference, there is shown an operational amplifier combined with a transistor and two resistors to form a full-wave rectifier. A problem with the showing in FIGS. 3 and 4 of the Miura et alii reference is that the mechanism for rectifying the positive-going portion of the AC signal is not similar to the mechanism for rectifying the negative-going portion of the AC signal. Consequently, such a circuit, although simple, will have a response which will not be uniform or symmetrical for positive and negative-going portions of the AC input signal, particularly at high frequencies.
Another approach to obtain full-wave rectification, described in U.S. Pat. No. 4,523,105 issued Jun. 11, 1985 to Jose et alii and entitled FULL-WAVE RECTIFIER CIRCUIT FOR SMALL SIGNALS includes an amplifier with a relatively complex degenerative voltage feedback arrangement to produce bidirectional output currents. In Jose et alii additional inversion of a portion of the bidirectional currents is required before the output currents can be combined to produce full-wave rectification. Thus, the mechanism for rectifying the positive-going half cycle is not the same as for the negative-going half cycle.
Therefore, there is still a need for a simple clipper circuit which has the stability and bandwidth advantages of a feedforward clipper, which has the dynamic range and linearity advantages of a feedback clipper, and which is easily reconnected to respond either to positive portions of an input signal or to negative portions of that input signal with the same predictable gain in either connection.
Furthermore, there is a need for circuits which are suitable for rectifying AC signals whose amplitudes may be a fraction of a volt and which can then use the rectified (or detected) signal to produce a direct current (DC) control voltage for controlling, for example, the gain of an amplifier.